1. Field of the Invention
This invention is directed generally to composite rubber materials in which a rubber vulcanizate and a thermosetting urethane elastomer are bonded or otherwise laminated together into an integral structure. More particularly, the invention relates to a method for such bonding.
2. Description of the Prior Art
Rubber products resulting from vulcanization of naturally occurring or synthetic rubbers find wide application in tires, belts, hoses, rolls and the like. Such products are available with various physical qualities which, however, are dependent upon the class of rubbers and when type of additives. It has been found that the vulcanized rubbers are combined with urethane elastomers into adhesively composite form, they can be diverse with respect to their physical characteristics. However, those vulcanizates do not firmly adhere to different materials.
To cope with the above problem of reduced adhesion, several methods have been proposed. One such method involves the oxidative treatment of a rubber vulcanizate on an outer side with a strong oxidant such as concentrated sulfuric acid, potassium permanganese or potassium bichromate, followed by washing of the oxidant with water and by subsequent flexing of the vulcanizate so as to cause fine or minute cracks over the treated surface. Another method uses isocyanate iodide for surface treatment of a similar vulcanizate as disclosed for instance in Japanese Patent Publication (JP-B) 51-31839. As a still further method JP-B 60-31217 teaches treating a surface of a vulcanized rubber with an organic type halogen compound having in the molecule a ##STR2## linkage where X represents a halogen atom and also with a dioxime compound.
The first-mentioned method has the drawback that the use of a strong oxidant requires meticulous care and oftentimes deteriorates the vulcanizate at or under the treated surface, inviting insufficient adhesion to the mating material. The second-mentioned method is undesirable for commercial application because the isocyanate iodide compound is of a special class requiring careful handling. The last-mentioned method lacks sufficient working life after surface treatment, entailing poor bondability.